A man who left his toddler son strapped into a car for seven hours alone on a boiling hot day has been found guilty of leaving him to die on purpose.

Justin Ross Harris told police after pulling his lifeless son from the car in Georgia, US, that it was an terrible accident.

However, police found the death suspicious - from the short drive during which Harris said he forgot about his son to the fact he returned to his SUV once without noticing the boy.

After a month-long trial and four days of deliberations, a jury on Monday sided with authorities and convicted 35-year-old Harris of malice murder - concluding not only that he should be held criminally responsible, but that he left 22-month-old Cooper to die on purpose.

Harris said he forgot to drop his son off at day care on June 18 2014, instead driving straight to his job as a web developer for Home Depot, not realising Cooper was still in his car seat.

Harris told police he did not notice Cooper until he left work for the day to go to a movie.

Soon afterwards, investigators found evidence that Harris was having sexual relationships - both online banter and in-person affairs - with numerous women, including a prostitute and a teenager.

Prosecutors charged Harris with malice murder, saying he intentionally killed his son in order to escape the responsibilities of family life.

Prosecutors ultimately prevailed with their argument that Harris must have known Cooper was in the car.

Harris drove less than two minutes to work after strapping the child into his car seat when they finished breakfast at a Chick-fil-A restaurant just over a half-mile from Harris's office.

Carpark surveillance video showed Harris also went to his car after lunch and tossed in some light bulbs he had purchased, though he never got inside.

Detectives testified Harris seemed too calm when answering their questions hours after his son died.

Defence lawyers said Harris was responsible for his son's death, but insisted it was an accident rather than a crime.

Friends and family members testified he was a devoted and loving father, and the jury watched video clips of Harris trying to teach Cooper to say "banana" and letting the boy strum his guitar. The joyous moments had some jurors laughing aloud.

Harris's ex-wife, Leanna Taylor, also came to his defence. She divorced him in March and bitterly told the jury that Harris "destroyed my life."

But she testified he was a loving father who, regardless of how unhappy he may have been in their marriage, would not have harmed their son on purpose. Ms Taylor was not in the courtroom on Monday. Harris was alone, except for his lawyers, as the verdict was read.

An attorney for Taylor, Lawrence Zimmerman, said they were disappointed in the verdict.

"Clearly it is our belief that this was not done with any malice," Mr Zimmerman said in an emailed statement.

Jurors seen leaving the courthouse on Monday declined to speak with reporters. Mr Boring said he had spoken with some of them, and was told they were nearly unanimous when they began deliberations last week. He said they wanted to make sure and review the evidence, taking four days to deliver a verdict.

Prosecutors said Harris left online clues to murderous intentions.

Evidence showed that minutes before Harris locked the car door on his boy, he sent an online message: "I love my son and all, but we both need escapes."

Five days earlier, Harris watched an online video in which a veterinarian sits inside a hot car to show it reaches 116 degrees in half an hour.

Harris was also found guilty of sending sexual text messages to a teenage girl and asked for nude photos of her pubic area.

The girl testified Harris knew she was in high school the months they swapped sexual banter when she was 16 and 17, and Harris several times sent her photos of his penis. He was asking for a photo of her breasts the day Cooper died.

Harris moved to Georgia from Tuscaloosa, Alabama, in 2012.

He lived in the Atlanta suburb of Cobb County, which is also where Cooper died. Because of intense pretrial publicity surrounding the case, the judge agreed to relocate Harris's trial 275 miles away in the coastal port city of Brunswick.

Harris's lawyers said he never spoke of the verdict when they met with him in a holding cell after the case ended.

"Instead he recognised he can now begin the grieving process he's not been able to go through the last two-plus years," lawyer Maddox Kilgore said. "He talked about Cooper and how much he misses him."

Harris held a flat stare that showed little emotion as the verdict was read. He was found guilty of all eight criminal counts against him.

The malice murder charge alone carries a sentence of life in prison, either with or without the possibility of parole. The trial judge scheduled a sentencing hearing for December 5.

"I believe categorically, unequivocally, that justice was served today," Cobb County District Attorney Vic Reynolds told reporters outside the courthouse, though he added "it's not a verdict we celebrate".

Harris's defence team vowed to appeal the verdict and to seek a new trial as well.

"From the moment we met Ross Harris, we've never once wavered in our absolute belief that he's not guilty of what he's been convicted of," defence attorney Maddox Kilgore told reporters.